Magnetic properties of reactively sputtered Fe_1-xO and Fe ₃O₄ thin films

We have investigated the magnetic properties of Fe₃O₄ thin films with thicknesses below 1000 Å. Previous reports had indicated an anomalous decrease in the saturation magnetization of Fe₃O ₄ films with thicknesses below 700 Å. The films were prepared by reactive rf magnetron sputtering and effects of film thickness, oxygen flow rate, and substrate temperature were examined. A region of acceptable magnetic properties, which do not vary with film thickness, for Fe₃O ₄ films has been found at both RT and 400°C. Within the processing region for single-phase Fe₃O₄, the magnetic properties did not vary significantly with oxygen partial pressure but did change with substrate temperature. The RT films are characterized by low saturation magnetization, 250 emu/cm³, and modest coercivity, 250 Oe. The films at 400°C have a higher saturation magnetization, 330 emu/cm ³, but also a higher coercivity, 450 Oe. The increased saturation magnetization at higher temperature results from the higher film density and lower degree of cation disorder; the higher coercivity at 400°C is anomalous. In addition, at RT we have discovered a processing region where ferrimagnetic wüstite, Fe_1-xO, is formed. The Fe_1-xO films have good soft magnetic properties and warrant further study.